The positioning and deployment of medical devices within a patient is a common, often-repeated procedure of contemporary medicine. Such medical devices or implants are used for innumerable medical purposes, including the reinforcement of recently re-enlarged lumens, or the replacement of ruptured vessels.
Coatings are often applied to the surfaces of these medical devices to increase their effectiveness. These coatings may provide a number of benefits including reducing the trauma suffered during the insertion procedure, facilitating the acceptance of the medical device into the target site, and improving the post-procedure effectiveness of the device.
Coating medical devices also provides for the localized delivery of therapeutic agents to target locations within the body, such as to treat localized disease (e.g., heart disease) or occluded body lumens. Such localized delivery of therapeutic agents has been achieved using medical implants which both support a lumen within a patient's body and place appropriate coatings containing absorbable therapeutic agents at the implant location. This localized drug delivery avoids the problems of systemic drug administration, such as producing unwanted effects on parts of the body which are not to be treated, or not being able to deliver a high enough concentration of therapeutic agent to the afflicted part of the body. Localized drug delivery is achieved, for example, by coating expandable stents, coronary stents, stent grafts, vascular grafts, catheters, balloon catheters, balloon delivery systems, aneurism coils, guide wires, filters (e.g., vena cava filters), intraluminal paving systems, implants and other devices which directly contact tissue, e.g., the inner vessel wall, with the therapeutic agent to be locally delivered.
The delivery of expandable stents is a specific example of a medical procedure that may involve the deployment of coated implants. Expandable stents are tube-like medical devices that often have a mesh-like patterned structure designed to support the inner walls of a lumen. These stents are typically positioned within a lumen and, then, expanded to provide internal support for it. Because of the direct contact of the stent with the inner walls of the lumen, stents have been coated with various compounds and therapeutics to enhance their effectiveness. The coating on these medical devices may provide for controlled release, which includes long-term or sustained release, of a biologically active material.
Aside from facilitating localized drug delivery, medical devices are coated with materials to provide beneficial surface properties. For example, medical devices are often coated with radiopaque materials to allow for fluoroscopic visualization during placement in the body. It is also useful to coat certain devices to achieve enhanced biocompatibility and to improve surface properties such as lubriciousness.
Conventionally, coatings have been applied to medical devices by processes such as dipping or spraying. For example, spray coating generally involves spraying the coating substance onto the device. Dipping, or spin-dipping, generally involves dipping a (static or spinning) device into a coating solution to achieve the desired coating. Another example, electrostatic fluid deposition, typically involves applying an electrical potential difference between a coating fluid and a target to cause the coating fluid to be discharged from the delivery point and drawn toward the target. Common to these processes is the need to apply the coating in a manner to ensure that a uniform, robust coating of the desired thickness is formed on the medical device or stent.
These conventional coating processes are often, however, indiscriminate and/or difficult to control. For example, dipping can result in non-uniform application of the coating to the device because gravity and longer exposure time may cause more coating to be applied at one end or region of the device, thus the coating may be thicker at one end. With respect to conventional spray coating and electrostatic spray deposition, empirical experience has shown that the spray plume stability of a spray nozzle used in both spraying and electrostatic spray coating is affected by vibration. The vibration may come from several sources, including, for example, fans and motors proximate to the spray plume and potential pressure variances within the coating fluid supply line which may cause flow interruptions or shock waves. Instability in the spray plume caused by vibration can cause variability in coating thickness and weight and reduce manufacturing reproducibility. Additionally, the venturi effect of the atomizing fluid may pull more coating fluid from the spray nozzle, which further limits controllability over the spray plume.
In addition, conventional spray nozzles typically provide a wide range of spray droplet sizes, which increases coating variance. Further, conventional spray nozzles typically have a dome-shaped nozzle geometry which limits controllability of spray droplet size as the coating material is pulled directly from the orifice due to the venturi effect of the atomizing fluid.
Thus, coating thickness can vary significantly on an individual target-to-target basis. Such variability could be detrimental to obtaining consistent coating distribution and thickness on the target, making it difficult to predict the dosage of therapeutic that will be delivered when the medical device or stent is implanted.
There is, therefore, a need for a cost-effective method and apparatus for coating the surface of a target or medical device that can provide one or more benefits such as increasing coating uniformity, improving manufacturing repeatability, minimizing waste in coating medical devices with expensive active agents, and/or permitting precise control of coating deposition rates, leading to highly efficient production systems.
The assignee of the current patent application is also the assignee of another patent application directed to resolving some of the problems noted above. The disclosure of U.S. patent application Ser. No. 10/774,483, filed Feb. 10, 2004, and entitled, “Apparatus and Method for Electrostatic Spray Coating of Medical Devices,” is hereby incorporated herein by reference.